1. Field of the Invention
The present invention relates to an image forming apparatus and control method thereof and, more particularly, to an image forming apparatus and control method thereof for applying multi-level error diffusion processing to an image forming apparatus which forms a visible image by exposure scanning and development on an image carrier and transfers it to a print medium.
2. Description of the Related Art
Pseudo halftoning has conventionally been used to represent input multi-level data by data of a smaller number of levels. Pseudo halftoning is an image processing method for representing multiple tone levels more naturally when input multi-level image data is expressed as, for example, a two-level image using only white and black dots. A representative of the pseudo halftoning is an error diffusion method (see, for example, “An Adaptive Algorithm for Spatial Gray Scale” in society for Information Display 1975 Symposium Digest of Technical Papers, 1975, p. 36). However, dot stability sometimes degrades when the error diffusion method is applied to an electrophotographic image forming apparatus such as a laser printer.
As a method for solving this problem, there is proposed pseudo halftoning of executing AM screening after FM screening to adjust the dot size and dot density (see, for example, Japanese Patent Laid-Open No. 2002-118748). This pseudo halftoning is hardly influenced by data noise (artifact) and variations in a printer.
However, pseudo halftoning by a conventional error diffusion method cannot achieve satisfactory graininess and dot stability, compared to screening.